The use of birefringent electrooptic crystal materials in the construction of electrically controlled optical modulators is well known. Various configurations of electrooptic modulators using the r.sub.63 electrooptic coefficient in crystal materials of crystallographic point group 42 m (KH.sub.2 PO.sub.4 and its isomorphs, for example) are described in U.S. Pat. No. 3,429,636 (Wentz) and in U.S. Pat. No. 3,402,002 (Eden). Eden describes electrooptic modulators operated by electric fields applied parallel to the direction of light passage through the devices (known as longitudinal-mode) and electrooptic modulators operated by electric fields applied perpendicular to the direction of light passage through the devices (known as transverse-mode). Other forms of transverse-mode electrooptic modulators which depend upon the r.sub.41 electrooptic coefficient of crystals of point group 42 m have been described by M. Dore in an article entitled "A Low Drive-Power Light Modulator Using a Readily Available Material ADP", IEEE Journal of Quantum Electronics, Volume QU-3, No. 11, November 1967 and R. D. Compton in an article entitled "The Promising World of Electro-Optical Modulators", Electro-Optical Systems Design, Volume 1, No. 2, September, 1969.
Of particular interest are the paired-crystal electrooptic modulator configurations disclosed in U. S. Pat. No 3,402,002 for using the r.sub.63 electrooptic coefficient in crystal materials of point group 42 m in the transverse mode, since these configurations permit the adjustment of design parameters to allow the use of lower operating voltages than are required for the operation of single element, longitudinal-mode devices. One embodiment (FIG. 3) of the invention disclosed in the foregoing patent requires each of the elements in a crystal pair to have an electric field applied in a direction that is perpendicular to the direction in which the electric field is applied to the other element of the crystal pair. This is a somewhat inconvenient arrangement that makes it impossible to contain both crystal elements between a parallel pair of electrodes. Another embodiment (FIG. 4) requires the use of a 90.degree. optical rotator, for the wavelength of light of interest, between a pair of similarly oriented crystal elements to permit the use of electrodes that are positioned parallel in pairs for the two elements. It must be noted that the crystallographic axes are incorrectly shown in the foregoing FIG. 4. The orientations of the axes illustrated will produce no first-order electrooptic effect from either the r.sub.63 or the r.sub.41 electrooptic coefficient. Referring to FIG. 6 of this patent, it is apparent that the crystallographic axes shown in FIG. 4 should be in a position rotated by an angle of 90.degree. about the direction of the light beam, in the same sense for both crystal elements For light that is substantially collimated, it is well known in the art that a halfwave plate chosen for the wavelength of interest may be used in place of the optical rotator in the structure shown in FIG. 4.
The crystallographic axes in the two crystal elements in the structure shown in FIG. 4 may be chosen to permit the application of identical electrical polarities to the parallel pairs of electrodes while having the electrooptically induced retardations additive for polarized light passing through both elements as shown. This may be accomplished, for example, by fabricating either one of the crystal elements in FIG. 4 such that its crystallographic orientation is rotated by an angle of 90.degree. about the Z axis when referred to the corrected orientations of the axes discussed above.
The advantages of mounting the two crystal elements of a matched pair between two supporting electrodes include ease of crystal alignment, simplification of the supporting structure, the option of bringing the elements close together (limited only by the thickness of the interposed element) to minimize space requirements, and the relative ease of accommodating the transfer of heat to maintain the elements at substantially equal temperatures which is required in the birefringence-compensated multi-crystal design.
The elimination of the halfwave plate or optical rotator between the paired crystal elements requires, in present devices, that the elements be oriented so that the applied electric field in each element is perpendicular to the electric field in the other element as shown in FIG. 3 of U.S. Pat. No. 3,402,002. This configuration prevents the use of paired parallel electrodes and also requires that the crystal elements be separated by a significant space to avoid electric shorting or arcing The larger spacing between the elements tends to increase the difficulty of maintaining essentially identical temperatures for the elements through the use of thermal links
As lasers have increased in size, with larger beam apertures and greater optical power levels, the construction of multi-crystal birefringence-compensated Pockels modulators has been increasingly constrained and limited by the designs set forth in U.S. Pat. No. 3,402,002 or others similar thereto. For the larger beam apertures required to accommodate the increased optical power levels produced by the larger lasers, the use of rectangular apertures has evolved because of considerations such as heat transfer from the modulating crystals. Provision for the required dissipation of the energy absorbed from the optical beams is simplified by the shorter thermal paths, from the crystal material to the electrodes, made possible by the use of rectangular crystal elements For a rectangular aperture that fills the crystal elements, the structure shown in FIG. 3 of U.S. Pat. No, 3,402,002 permits full electrooptic function of only one of the two paired crystal elements, since, for one of the pair, the distance between the electrodes for that element in FIG. 3 would be made significantly larger than the distance between the electrodes for the other electrooptically functional element. The increase in distance between the electrodes results in a proportional reduction in electric field, for a given applied electric potential, in the element having its electrooptic function compromised. The relative electrooptic response of the assembly is reduced by a factor that approaches 0.5 for this structure compared to the response available if both crystal elements were fully functional electrooptically.
The electrooptic functioning of both crystal elements of the rectangular matched pair may be obtained in current devices by using the configuration illustrated in FIG. 4 of U.S. Pat. No. 3,402,002 for a rectangular aperture, with the longer dimension of the aperture parallel with the electrodes. This approach, however, requires the use of an additional component (an optical rotator or halfwave plate) having an optical aperture as large as that of the electrooptic elements. An optical rotator (typically of the highest quality optical quartz) or a halfwave plate having a large aperture is relatively expensive, and the use of such would add two additional optical surfaces to the operating system. Additional surfaces could cause additional optical reflections and add distortions to the optical wavefronts passing through the system.
Because of the inherent problems associated with the prior art devices, it has become desirable to devise means for energizing a plurality of crystal elements supported between a parallel pair of electrodes without the use of an optical rotator or halfwave plate between the crystal elements.